Extending wave propagation along muscle fibers originates from early local contraction at the end-plate region

We investigated the propagation of extending waves along twitching muscle fibers triggered by the internal shortening of early local contraction in the end-plate region. Bullfrog sartorius muscles were minimally stimulated, and the displacement of carbon particles attached to the muscle surface was captured using a high-speed camera. We found an extending wave along the fiber at a velocity of 5.35 ± 0.33 m·s⁻¹, faster than the conduction of action potentials at 3.04 ± 0.31 m·s⁻¹. Local compression of the muscle surface blocked the propagation of the extending wave, indicating its mechanical nature. Muscle stretching increased the extending wave velocity. These findings provide direct evidence that mechanically transmitted extending waves originate from early local contractions in the end-plate region and propagate along muscle fibers ahead of the contraction wave.